The present invention relates to an apparatus and method for sensing oxygen levels of a fetus.
During normal labor, the fetal head engages in the pelvis. Coincident with the mother pushing, uterine contractions, and dilatation of the cervix, the fetus is delivered.
In utero, the fetus receives its oxygen through the placenta. When the uterus contracts, the oxygen supply to the fetus is reduced until the uterus relaxes between contractions. In severe cases, this may result in fetal asphyxia, an impaired or absent exchange of oxygen and carbon dioxide on a ventilatory basis in the fetus. In extreme cases, fetal asphyxia may be associated with fetal cerebral injury.
Under the current standard of care, it is common for the heart rate of the fetus to be monitored during labor. In many instances, this fetal heart rate monitoring is performed through the cervix or the abdominal/uterine wall of the mother. Currently, the change in the fetus's heart rate relative to the uterine contractions is used to try to identify fetuses at risk. This current standard of care has been essentially, unsuccessful. "Current methods of intrapartum surveillance have made little impact in fetal mortality and morbidity while leading to increased caesarean section rates." Peoples, D. M., "Cerebral Hemodynamics and Oxygenation in the Fetus. The Role of Intrapartum near-infrared spectroscopy," Clinics in Perinatalogy September 1997 Vol. 24(3), pp. v. & 547-65. As a result research has been directed to better determine fetal oxygen levels.
One example of a known apparatus and method for sensing fetal oxygen levels is disclosed in U.S. Pat. No. 5,813,980 to Levinson et al., and assigned to Nellcor Puritan Bennett Incorporated. This method and apparatus is an optical method, which uses two wavelengths of light to determine the level of oxygen in pulsatile blood. This method is incapable of measuring fetal oxygen levels in non-pulsatile, or venous, blood.
U.S. Pat. No. 5,813,980 also teaches that there are two known types of fetal sensors: presenting part sensors and beyond the presenting part sensors. "Presenting part" refers to the region of the cervical os. "Beyond the presenting part" falls with in the uterus and extends out to the cervical os. Beyond the presenting part sensors can typically use the uterine wall to bias the sensor against the fetus. Presenting part sensors, on the other hand, cannot rely on the bias of the uterine wall, and may require positive attachment. Both known types of sensors, beyond the presenting part and presenting part, are placed through the cervix and require direct contact with fetal tissue. Since the sensor for both types must be placed in contact with fetal tissue, it interferes and partially obstructs the birth canal. Direct contact and interference with the fetus increases the risk of incidental injury to the fetal tissue from instrumentation and sensors. Further the protective amniotic fetal membranes would have to be ruptured to use this device. Finally, both known types of sensors can only be used when the fetal tissue is at or near the cervix. Thus, using either sensor it is not possible to measure the fetal oxygen level prior to labor.
In addition to monitoring the fetus, during labor, a catheter, typically a Foley catheter, is sometimes inserted into the mother's bladder for the purposes of evacuating urine. A Foley catheter inflates a balloon through an inflation lumen. The urine drains through the drainage lumen. The inflated Foley balloon keeps the catheter from sliding out of the bladder. Foley catheters are incapable of measuring fetal oxygen levels or other parameters. Thus, using previously known methods, to simultaneously evacuate urine and monitor fetal oxygen levels required the use of at least two separate devices.